Electrical feedthroughs serve the purpose of providing an electrical circuit path extending from the interior of a hermetically sealed container to an external point which is electrically insulated from the container itself, such as in an electrochemical cell. Many such feedthroughs which provide such an electrical path are known in the art. A principal difficulty with electrical feedthroughs lies in the continuing miniaturization of electrochemical cells, and the resulting requirements for correspondingly smaller feedthrough dimensions. For example, various electrochemical cell designs require low profile feedthroughs, i.e. feedthroughs having a very low height. Providing an adequate electrical insulation system for such low profile feedthroughs is difficult.
Various electrical feedthroughs, including the feedthrough described in the present application, are employed in conjunction with electrochemical cells to provide for an electrical path to an anode or cathode within the cell. Such electrochemical cells are used in implantable medical devices, such as cardiac pacemakers, and in conjunction with complex miniaturized electronic circuits. Electrochemical cells, such as the lithium/iodine cell as described in U.S. Pat. No. 4,166,158 entitled "Lithium-Iodine Cell" and U.S. Pat. No. 4,460,664 entitled "Electrochemical Cell for Facilitating Hermeticity Leakage Testing of an Electrical Feedthrough," possess an active cathode which is capable of attacking many materials used in electrical feedthroughs and can migrate along electrical leads to cause circuit malfunction. Therefore, the insulating system of the feedthrough is of significant importance.
Electrical feedthroughs typically include a metal ferrule which retains an electrical lead, and a seal means, usually glass. The ferrule is attached, as by welding, to the casing or cover of an electrochemical cell container and has a portion which extends into the interior of the container, and a portion which extends exteriorly of the container. The lead, retained in the feedthrough and in the interior of the container, must be insulated or protected from the electrochemically active cathode material of the electrochemical cell; for example, as described in the aforementioned U.S. Pat No. 4,166,158.
In conventional feedthrough insulation systems and methods of making the feedthrough, for example, the height to diameter ratio of the feedthrough is typically about 2 or greater. The ferrule for the feedthrough is generally welded into the cover of the cell and then the lead is glassed into the ferrule. The ferrule typically has a shoulder on it for location during an injection molding process and also for welding purposes. After the lead is glassed into the ferrule, an insulator is injection molded onto the ferrule using the cover and/or shoulder of the ferrule for positioning the ferrule in the mold in the injection molding process. The insulator is typically a fluoropolymer. The cover and/or shoulder on the ferrule used for location orient the ferrule in the mold and create a sealed cavity required for injection molding the insulator onto the ferrule. A good seal between a mold and part is needed for the high injection pressures utilized. Feedthroughs that are molded discretely without covers, typically provide a shoulder of adequate size to create the sealed cavity between the part and the mold and to prevent the ferrule from moving around in the mold during the injection molding process.
In some designs of electrochemical cells, the ferrule cannot be welded to a cover prior to glassing the lead into the ferrule. Thus, in order to accomplish injection molding by conventional means, the ferrule requires a shoulder of adequate size that can be used for locating the ferrule in a mold and creating a seal with the mold during injection molding. However, because of the reduced height of the ferrules for particular designs, limited insulation distance is provided to electrically insulate the lead retained in the feedthrough from the cathode material of the electrochemical cell if such a shoulder of the ferrule is included. As a result, conventional locating and sealing of the ferrule in a mold using a ferrule shoulder is inadequate for providing an insulation system for the feedthrough when using injection molding and further, without an adequately sized shoulder or use of the cover to retain the ferrule in place during the injection molding process, the ferrule tends to undesirably move during the injection molding process. Although various other insulation techniques are available for providing the electrical insulation necessary for a feedthrough of an electrochemical cell, such as, for example, potting compounds, injection molding of a feedthrough is generally preferred.
For the above reasons, improvements in the design of the feedthroughs for electrochemical cells are needed, particularly to provide an adequate feedthrough insulation system for low profile electrochemical cells. The present invention, as described below, provides such improvement and overcomes the problems described above and those problems which will become apparent to one skilled in the art from the detailed description provided below.